My Illogical Theory
Here's meh theory. :3
I believe that they use Dark-types, as Dark-types mostly represent evil. In my boring English class we learnt that directors normally associate dull, black colours with evil people, and light, white colours will good people. As Infiniti stated, Poison-types are also shown because poison is deadly - to kill someone with poison would be evil!
Why they use cats of dogs, on the other hand - have you read these Pokemon's entries?
>Purrloin, the Devious Pokémon. Purrloin fools people into letting their guard down so it can steal their possessions. When angered, it uses its claws to fight back.
>Mightyena gives obvious signals when it is preparing to attack. It starts to growl deeply and then flattens its body. This Pokémon will bite savagely with its sharply pointed fangs.
And there are always Pokemon like Misdreavus and Drifloon, who are also evil in their Pokedex entries, but I think Misdreavus let's off a 'girly' feeling (That's why I love it. Lol. xD) and Drifloon is a balloon. A BALLOON.
So, yeah. xD
I think I've explained enough. :3
Hope I helped. :)